One essential problem in radio systems is the rapid variation of the properties of a radio channel with time. This relates especially to mobile systems, in which at least one of the participants in a connection is often mobile. The attenuation and impulse response of the radio channel then vary within a wide phase and amplitude range even thousands of times per second. The phenomenon is random by nature, so mathematically it can be described by statistical means. The phenomenon complicates the design of radio connections and the used devices.
There are many reasons for the variation in a radio channel. When transmitting a radio frequency signal from a transmitter to a receiver in a radio channel, the signal propagates along one or more paths, in each of which the phase and amplitude of the signal vary, which causes fades of different lengths and strengths in the signal. In addition, noise and interference from other transmitters also disturbs the radio connection.
A radio channel can be tested either under actual conditions or using a simulator that simulates the actual conditions. Tests conducted in actual conditions are difficult, because tests taking place outdoors, for instance, are affected for example by the weather and season that change all the time. Even measurements taken in the same place produce a different result at different times. In addition, a test conducted in one environment (city A) does not fully apply to a second corresponding environment (city B). It is also usually not possible to test the worst possible situation under actual conditions.
However, with a device simulating a radio channel, it is possible to very freely simulate a desired type of a radio channel between two radio devices in such a manner that the radio devices operate at their natural transmission rates, just like in an actual operating situation. FIG. 1 illustrates an example of a device for simulating a radio channel. The figure shows a first set of devices 100 to 108 and a second set of devices 110 to 118, and a channel simulator 120. The first set of transmitters 100 to 108 can comprise mobile phones, for instance, that through their antenna connectors are connected to the inputs of the channel simulator 120. The second set of devices 110 to 118 can in turn be receivers of base station equipment that are connected to the outputs of the channel simulator. The number of the first and second devices need not be the same. In the example of the figure, there are five devices in each set.
A channel simulator typically comprises several channel elements that are capable of simulating and modelling a desired channel type. The channel simulator of FIG. 1 comprises eight elements. Each element comprises both a radio frequency part and a baseband part. A signal can be fed to the input of the channel simulator either in radio frequency or baseband format. In the latter case, the radio frequency parts of the channel elements are bypassed. In the radio frequency part, a signal is converted to baseband, and the resulting baseband signal is forwarded to the baseband parts, in which the impact of the channel fade is added to the signal.
In prior-art solutions, the channel element forms a fixed unit. As in the case of FIG. 1, there may be situations during simulation, in which not all channel elements are used, because there are fewer channels to simulate than the device has capacity for.